Looking At The Transmission Of Optical Communication Cultural Studies Essay

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Optical communication is the transmission and/or reception of information using optical signals. Optical communication may use optical waveguides (e.g. fiber optic lines) or free space transmission to transfer optical signals. Optical communication is one of the newest and most advanced forms of communication by electromagnetic waves. In one sense, it differs from radio and microwave communication only in that the wavelengths employed are shorter (or equivalently, the frequencies employed are higher). However, in another very real sense it differs markedly from these older technologies because, for the first time, the wavelengths involved are much shorter than the dimensions of the devices which are used to transmit, receive, and otherwise handle the signals.

The advantages of optical communication are threefold. First, the high frequency of the optical carrier (typically of the order of 300,000 GHz) permits much more information to be transmitted over a single channel than is possible with a conventional radio or microwave system. Second, the very short wavelength of the optical carrier (typically of the order of 1 micrometer) permits the realization of very small, compact components. Third, the highest transparency for electromagnetic radiation yet achieved in any solid material is that of silica glass in the wavelength region 1-1.5 μm. This transparency is orders of magnitude higher than that of any other solid material in any other part of the spectrum. See also Electromagnetic radiation; Light.

Optical fiber communications

With the development of extremely low-loss optical fibers during the 1970s, optical fiber communication became a very important form of telecommunication almost instantaneously. For fibers to become useful as light waveguides (or light guides) for communications applications, transparency and control of signal distortion had to be improved dramatically and a method had to be found to connect separate lengths of fiber together.

Sadhana Prakash Shisode

B.E.(C omputer)N.M.U.Jalgaon

Computer Deptt. D.N.Patel COE Shahada

A/P-Shahada,Tal. Shahada,Dist.Nandurbar ,India

e-mail: sadhanapatil45@gmail.com.

The transparency objective was achieved by making glass rods almost entirely of silica. These rods could be pulled into fibers at temperatures approaching 3600°F (2000°C).

Reducing distortion over long distances required modification of the method of guidance employed in early fibers. These early fibers (called step-index fibers) consisted of two coaxial cylinders (called core and cladding) which were made of two slightly different glasses so that the core glass had a slightly higher index of refraction than the cladding glass. By reducing the core size and the index difference in a step-index fiber, it is possible to reach a point at which only axial propagation is possible. In this condition, only one mode of propagation exists. These single-mode fibers can transmit in excess of 1011 pulses per second over distances of several hundred miles. See also Waveguide.

The problem of joining fibers together was solved in two ways. For permanent connections, fibers can be spliced together by carefully aligning the individual fibers and then epoxying or fusing them together. For temporary connections, or for applications in which it is not desirable to make

Almost every major metropolitan area in the United States has a light-wave transmission system in service connecting telephone central offices. These systems typically operate at a wavelength of either 1.3 or 1.55 μm (where silicon fibers have a minimum loss). It is anticipated that light-wave systems will gradually be installed in the telephone loop plant-that is, the portion of the telephone plant which connects the individual subscriber to the telephone central office. See also Data communications; Facsimile.

Optical transmitters

In principle, any light source could be used as an optical transmitter. In modern optical communication systems, however, only lasers and light-emitting diodes are generally considered for use. The most simple device is the light-emitting diode which emits in all directions from a fluorescent area located in the diode junction. Since optical communication systems usually require well-collimated beams of light, light-emitting diodes are relatively inefficient. On the other hand, they are less expensive than lasers and, at least until recently, have exhibited longer lifetimes.

Another device, the semiconductor laser, provides comparatively well-collimated light. In this device, two ends of the junction plane are furnished with partially reflecting mirror surfaces which form an optical resonator. As a result of cavity resonances, the light emitted through the partially reflecting mirrors is well collimated within a narrow solid angle, and a large fraction of it can be captured and transmitted by an optical fiber.

Both light-emitting diodes and laser diodes can be modulated by varying the forward diode current.

Optical receivers

Semiconductor photodiodes are used for the receivers in virtually all optical communication systems. There are two basic types of photodiodes in use. The most simple comprises a reverse-biased junction in which the received light creates electron-hole pairs. These carriers are swept out by the electric field and induce a photocurrent in the external circuit. The minimum amount of light needed for correct reconstruction of the received signal is limited by noise superimposed on the signal by the following circuits. See also Photodiode.

Avalanche photodiodes provide some increase in the level of the received signal before it reaches the external circuits. They achieve greater sensitivity by multiplying the photo generated carriers in the diode junction. This is done by creating an internal electric field sufficiently strong to cause avalanche multiplication of the free carriers. See also Microwave solid-state devices; Optical detectors.

OPTICAL COMPUTING

The knowledge of some history of sciences is useful for understanding the evolution of a research domain, its successes and failures. Optical computing is an interesting candidate for a historical review. This research field is also named optical information processing, and now the terms of information optics or information photonics are frequently used, reflecting the evolution of the domain. Optical computing is approximately 60 years old and it is a well-defined domain with its own specialized conferences, sections in the scientific journals and its own research programs and funding. It was also very active worldwide and therefore it is impossible in the frame of a paper to describe all the research results. Numerous books were written on the subject, for example, the following books describe the state of the art of optical computing at the time of their publication in 1972 [1], in 1981-82 [2, 3], in 1989 [4], and in 1998-99 [5, 6]. Since optical computing is such a well defined field over such a long period of time, it is interesting to study its evolution and this study can be helpful to understand why some research domains were very successful during only a limited period of time while other have generated numerous applications that are still in use. From the beginning there Was a lot of questioning about the potential of optics for computing whereas there was no doubt about the potential and the future of electronics? Caulfield wrote in 1998 an interesting and enlightening paper on the perspectives in optical computing [7] where he discusses this competition Between optics and electronics and shows that there were three phases, first "ignorance and underestimation" of electronics then "awakening and fear inferiority" and now "realistic acceptance that optical computing and electronics are eternal partners". The purpose of this paper is to show a short history of optical computing from the origin until today. This historical overview will show that the first years generated a lot of enthusiasm regarding the potential of optics for information processing; this period was followed by a small slowdown before the golden age that started around 1980 until the beginning of the new century. Section 2 presents the basic principles of optical information processing, Section 3 gives a historical review of the research from the first years until 1980 and Section 4 describes the research activity from 1980 to 2004. Section 5 Shows the evolution of the domain until today.

Fundamentals of Optical Information

Optical information processing is based on the idea of using all the properties of speed and parallelism of the light in 2 Advances in Optical Technologies order to process the information at high-data rate. The information is in the form of an optical signal or image. The inherent parallel processing was often highlighted as one of the key advantage of optical processing compared to electronic processing using computers that are mostly serial. Therefore, optics has an important potential for processing large amount of data in real time. The Fourier transform property of a lens is the basis of optical computing. When using coherent light, a lens performs in its back focal plane the Fourier transform of a 2D transparency located in its front focal plane. The exact Fourier transform with the amplitude and the phase is computed in an analog way by the lens. All the demonstrations can be found in a book published in 1968 by Goodman [8] and this book is still a reference in the field. The well-known generic architecture of optical processors and the architectures of the optical correlators will be presented successively.

Optical Processor Architecture.

The architecture of a generic optical processor for information processing is given in Figure 1. The processor is composed by three planes: the input plane, the processing plane, and the output plane. The data to be processed are displayed in the input plane, most of the time this plane will implement an electrical to optical conversion. A Spatial Light Modulator (SLM) performs this conversion. The input signal can be 1D or 2D. An acousto-optic cell is often used in the case of a 1D input signal and 2D SLMs for 2D signals. The different types of 2D SLMs will be described later. In the early years, due to the absence of SLMs, the input plane consisted of a fixed slide. Therefore the principles and the potential of optical processors could be demonstrated but no real-time applications were possible, making the processor most of the time useless for real life applications. The processing plane can be composed of lenses, holograms (optically recorded or computer generated) or nonlinear components. This is the heart of the processing, and in most optical processors, this part can be performed at the speed of the light. A photo detector, a photo detector array or a camera composes the output plane where the results of the processing are detected. Figure 1 shows clearly that the speed of the whole process is limited by the speed of its slowest component that is most of the time the input plane SLM, since the majority of them are operating at the video rate. The SLM is a key component for the development of practical optical processors, but unfortunately also one of their weakest components. Indeed, the poor performance and high cost of SLMs have delayed the fabrication of an optical processor for real-time applications.

A New Start for Optics; the Rise of Optical Computing (1945-1980)

Information optics is a recognized branch of optics since the fifties. However, historically, the knife-edge test by Foucault in 1859 [20] can originate the optical information processing. Other contributors can be noted such as Abbe in 1873 who developed the theory of image formation in the microscope [21], or Zernike who presented in 1934 the phase contrast filter [22]. In 1946, Duffieux made a major contribution with the publication of a book on the use of the Fourier methods in optics [23]. This book was written in French and translated in English by Arsenault [24]. The work byMar´echal is another major contribution; in 1953, he demonstrated the spatial frequency filtering under a coherent illumination [9]. Optical computing is based on a new way of analyzing the optical problems; indeed, the concepts of communications and information theory constitute the basis of optical information processing. In 1952, Elias proposed to analyze the optical systems with the tools of the communication theory [25, 26]. In an historical paper [27], Lohmann, the inventor of the computer-generated holography, wrote "In my view, Gabor's papers were examples of physical optics, and the tools he used in his unsuccessful attempt to kill the twin image were physical tools, such as beam splitters. By contrast, Emmett (Leith) and I considered holography to be an enterprise in optical information processing. . . .In our work, we considered images as information, and we applied notions about carriers from communications and information theory to separate the twin image from the desired one.

The Future of Optical Information Processing as Seen in 1962.

In order to understand the evolution of optical computing, it is enlightening to see the topics of discussions in the early sixties. For example, in October 1962, a "Symposium on Optical Processing of Information" was held in Washington DC, cosponsored by the Information System Branch of the Office of Naval Research and the American Optical Company. About 425 scientists attended this meeting and Proceedings were published [52]. The preface of the proceedings shows that the purpose of this symposium was to bring together researchers from the fields of optics and information processing. The authors of the preface recognize that optics can be used for special-purpose optical processors in the fields of pattern recognition, character recognition, and information retrieval, since optical systems offer in these cases the ability to process many items in parallel. The authors continue with the question of a general-purpose computer. They write: "Until recently, however, serious consideration has not been given to the possibility of developing a general purpose optical computer. With the discovery and application of new optical effects and phenomena such as laser research and fiber optics, it became apparent that optics might contribute significantly to the development of a new class of high-speed general purpose digital computers". It is interesting to list the topics of the symposium: optical effects (spatial filtering, laser, fiber optics, modulation and control, detection, electroluminescent, and photoconductive) and data processing (needs, biological systems, bionic systems, photographic, logical systems, optical storage systems, and pattern recognition). It can be noted, that one of the speakers, Teager [53] from MIT, pointed out that for him the development of an optical general-purpose computer was highly premature because the optical technology was not ready in order to compete with the electronic computers. For him, the optical computers will have a different form than the electronic computers; they will be more parallel. It is interesting to see that this debate is now almost closed, and today, 47 years after this meeting, it is widely accepted that a general purpose pure optical processor will not exist but that the solution is to associate electronics and optics and to use optics only if it can bring something that electronic cannot do.

Optical Memory and the Memory of the Electronic Computers.

It is useful to replace the research on optical memories in the context of the memories available in the sixties. At this time the central memory of the computers was a core memory and compact memory cells were available using this technology. However, the possible evolution of this technology was limited. The memory capacity was low, for example, the Apollo Guidance Computer (AGC), introduced in 1966, and used in the lunar module that landed on the moon, had a memory of 2048 words RAM (magnetic core memory), 36864 words ROM (core rope memory), with16- bit words. In October 1970, Intel launched the first DRAM (Dynamic Random Access Memory) the Intel 1103 circuit. This chip had a capacity of one Kbit using P-channel silicon gate MOS technology with a maximum access time of 300 ns and a minimum write time of 580 ns. This chip killed the core memory. Compared to the memories that were available, optical memories had two attractive features: a potential high density and the possibility of parallel access. Already in 1963, van Heerden developed the theory of the optical storage in 3D materials [54]. In 1968 the Bell Labs constructed the first holographic memory [55] with a holographic matrix of 32 by 32 pages of 64 by 64 bits each. In 1974, d'Auria et al. fromThomson-CSF [56] constructed the first complete 3D optical memory system storing the information into a photorefractive crystal with angular multiplexing and achieving the storage of 10 pages of 104 bits. Holographic memories using films were also developed in the US. Synthetic holography has been applied to recording and storage of digital data in the Human Read/Machine Read (HRMR) system developed by Harris Corporation in 1973 for Rome Air Development Center [57].

. Optical Fourier Transform Processors, Optical Pattern Recognition.

Optical pattern recognition was from the beginning a prime choice for optical processing since it was using fully the parallelism of optics and the Fourier transform properties. The book edited by Stark [3] in 1982 gives a complete overview of the state of the art of the applications of optical Fourier transforms. It can be seen that coherent, incoherent; space-invariant, space variant; linear, nonlinear architectures were used for different applications. Hybrid processors, optical/digital emerged as a solution for practical implementation and for solving real problems of data processing and pattern recognition. For example, Casasent, who was very active in this field, wrote a detailed book chapter [58] with a complete review of hybrid processors at the beginning of the eighties. All these processors gave very promising results. Almost all the proposed processors remained laboratory prototypes and never had a chance to replace electronic processors, even if at that time, electronics and computers were much less powerful. There are many reasons for this; the number of applications that could benefit from the speed of optical Advances in Optical Technologies 7 processors was perhaps not large enough, but the main reason was the absence of powerful, high speed, high quality, and also affordable SLMs, for the input plane of the processor as well as for the filter plane. Optical processors are also less flexible than digital computers that allow a larger number of data manipulations very easily.

Optical Computing Golden Age (1980-2004)

This period of time could be called the optical computing golden age. There was a lot of enthusiasm in the field, the future looked very bright, there was funding for the programs on the topic and the research effort was very intensive worldwide. Every years, several international conferences were organized by different international societies on subjects related to optical computing. The journals had frequently a special issue on the topics and Applied Optics had every 10th of month an issue entitled "Information Processing". The research was very fruitful in all the domains of optical information processing including theoretical work on algorithms, analog and digital computing, linear and nonlinear computing. Optical correlators for real applications were even commercialized. However, around 2000, we could feel that the interest for the subject started to decline. The reasons are multiple, but the evolution of digital computers in term of performance, power and also flexibility can be pointed out. They are also very easy to use even for a nonspecialist. It is impossible to list here all the work carried out in thedomain from 1980 to 2004. Several books give the state of theart of the domain at the time of their publication [4, 6, 59]. In the following, we will describe only some aspects of the research during this period, and we apologize for some important results that may be missing. The purpose is to give to reader an idea of the evolution of the domain during this quarter century.

From Computer Generated Holograms to Diffractive Optical Elements.

CGHs are important components for optical processing since they can process the information. The first CGHs were mostly cell-oriented since these methods were well adapted to the power of the computers with a small memory capacity and to the technology of the printers of this time. In the eighties, the technological landscape has changed, more powerful computers with a larger memory capacity were available, e-beam writers were more commonly used. Therefore new encoding methods, the point-oriented methods, were developed in order to achieve high quality and high diffraction efficiency optical reconstructions of the CGHs. First, the error diffusion algorithm, used for printing applications, was adapted to encode CGHs where it was possible to separate the noise from the desired pattern in the reconstruction plane [60]. Then, iterative algorithms were proposed and the best known are the Direct Binary Search (DBS) algorithm proposed by Seldowitz et al. in 1987 [61] and the Iterative Fourier Transform Algorithm (IFTA) proposed by Wyrowski and Bryngdahl in 1988 [62]. The CGHs encoded with these algorithms produce a reconstruction with a high Signal to Noise Ratio and high diffraction efficiency, especially in the case of pure phase CGHs. Later some refinements were proposed, for example the introduction of an optimal multicriteria approach [63, 64]. It should be noted that these iterative methods are still used. In the nineties, the main progress concerns the fabrication methods with the use of lithographic techniques allowing the fabrication of high precision phase only components etched into quartz. The name Diffractive Optical Elements (DOEs) that includes the CGHs is now used and reflects this evolution. Thank to the progress in lithography, submicron DOEscan be fabricated such as a polarization-selective CGH [65], artificial dielectrics [66], a spot generator [67]. The state of the art of digital nano-optics can be found in Chapter 10 of the book written by Kress and Meyrueis [68]. The nano structures fabrication required new studies of the diffraction based on the rigorous theory of diffraction instead of the scalar theory of diffraction [69]. Several books give a complete overview of the field of DOEs and their applications [68, 70] and a very complete paper on the evolution of diffractive optics was published in 2001 by Mait [71].

The Maturity of Spatial Light Modulators.

Since the availability of SLMs was an important issue for the success of optical information processing, a lot of effort has been invested after 1980 into the development of SLMs fulfilling the optical processors requirements in terms of speed, resolution, and size and modulation capability. A paper written by Fisher and Lee gives the status of the 2D SLM technology in 1987 [72] and shows that, at this time, the best feasible SLM performance values are found to include: about 100 Ã- 100 resolution elements, 10-Hz framing rates, 1-s storage, less than 50 μJ/cm2 sensitivity, five-level dynamic range, and 10-percent spatial uniformity. Updated reviews of the state of the art of SLMs is given in a book edited by Efron [73] in 1995 and in several special issues of "Applied Optics" [74-77]. More than 50 types of SLM have been introduced in the eighties and nineties [78]. Many different SLMs have been proposed and many prototypes fabricated-for example, besides liquid crystal SLMs, magneto-optic SLMs [79, 80], multiple quantum wells devices (MQW) [81], Si PLZT SLMs[82] and Deformable Mirror Devices [83, 84]. However very few of these SLMs have survived. Therefore, today, among the SLMs commercially available, mostly for displaypurpose, two technologies prevail: liquid crystal technology and Digital Micro mirrors Devices DMD (MEMS based technology). There are different types of liquid crystal SLMs. Twisted nematic liquid crystal SLMs are commonly used and their theory and experimental characterization show an amplitude and phase coupled modulation [85] as well as an operating speed limited to the video rate. Ferroelectric liquid crystal SLMs can reach a speed of several kilohertz, but most of the devices on the market are binary bistable devices 8 Advances in Optical Technologies that consequently limit the applications. Although it is not so commonly known, analog amplitude only modulation is possible with specific ferroelectric SLMs [86]. Nematic liquid crystal or Parallel Aligned liquid (PAL) crystal SLMs produce a pure phase modulation that can exceed 2Ï€. They are particularly attractive for applications requiring a high light efficiency such as dynamic diffractive optical elements. Their speed can reach 500 Hz [86]. The matrix electrically addressed SLMs using twisted nematic liquid crystal have progressed considerably. Around 1985, the small LC TV screens were extensively evaluated [87, 88], but their poor performance (phase nonuniformity, limited resolution, etc.) limited their use for optical computing; then VGA, SVGA, and XGA resolution SLMs were introduced in video projectors and these SLMs extracted from the video projectors were widely characterized [89] and integrated into optical processors. During the same period, high performanceoptically addressed SLMs were fabricated, for example, the PAL SLM from Hamamatsu [90]. Now high resolution Liquid Crystal on Silicon (LCoS) SLMs are commercially available, for example an pure phase LCoS SLM with 1920 Ã- 1080 pixel resolution is commercialized [91]. All these SLMs must be characterized very precisely and numerous papers were published on the subject [92-95]. In conclusion, today, for the first time since the origin of the optical processors, commercially available SLMs are fulfilling the requirements in terms of speed, modulationcapability, and resolution. The applications of SLMs are numerous, for example, recent papers have reported different applications of LCoS SLMs, such as pulse shaping [96], quantum key distribution [97], hologram reconstruction[98], computer generated holograms [99], DOEs [100], optical tweezers [101], optical metrology [102].

Optical Memories.

In a parallel optical computer, a parallel access optical memory is required in order to avoid the bottleneck between the parallel processor and the memory. Therefore the research for developing a 3D parallel access optical memory was very active in the last two decades of the last century. Different architectures using different technologies were proposed. For example, Marchand et al. constructed in 1992 a motionless-head parallel readout optical-disk system [103] achieving a maximum data rate of 1.2Gbyte/s. Psaltis from Caltech developed a complete program of research on 3D optical holographic memories using different materials such as photorefractive crystals. Inthe frame of this program, Mok et al. achieved to store 10000 holograms of 440 by 480 pixels [104] into a photorefractive crystal of 3 cm3. IBM was also very active into the field of holographic memory [105] and two important programs of the Darpa were carried out in the nineties: project PRISM (Photorefractive Information Storage Material), and project HDSS (Holographic Data Storage Systems). All the information on these holographic memories can be found in a book [106]. Several start-up companies were created for developing holographic memories and most of them disappeared. However one of them, In Phase Technologies, is now commercializing a holographic WORM disk memory system using a photopolymer material [107]. Other types of optical memories were investigated such as a two-photon memory [108], spectral hole burning [109].Today the holographicmemory is still seen as a candidate for the memory of the future, however the problem of the recording material is not yet solved; particularly there is no easy to use and cheap rewritable material. The photopolymers can only be written once and despite all the research effort photorefractive crystals are still very difficult to use and expensive.

. Optical Information Processing, Optical Pattern Recognition.

The last two decades of the last century were a very intensive period for the research in optical processing and optical pattern recognition. All the aspects of these processors were investigated and the research progressed remarkably. One key element in an optical correlator is the reference filter, and important part of the research concentrated on it. The correlation is shift-invariant but is scale-variant and orientation-variant. Therefore several solutions, using for example, Synthetic Discriminant Function (SDF) were proposed to overcome this drawback [110-112]. Beside the classical matched filter, several other improvements have been presented [113-117]. A large amount of work has been carried out to enhance the discrimination of the target in a complex scene [118]. The architecture of the JTC was also studied extensively, particularly by Javidi who proposed several improvements such as the nonlinear JTC [119-122]. A very large number of processors were constructed taking advantage of the progress of SLMs and of the theoretical work on the filters and on the architectures. Some of these processors stayed in the laboratories while some others were tested for real applications. Regarding the large number of optical processors that were constructed during this period of time, it is impossible to list them all in the frame of this paper. A book, written in French, by Tribillon gives a very complete state of the art of the optical pattern recognition in 1998 [5]. The book edited in 1999 by Yu and Yin give also a complete overview on the topic [123]. Therefore, you will find here only, some examples of the optical processors developed between 1980 and 2004.In 1982, Cleland et al. constructed an optical processor for detecting tracks in a high-energy physics experiment. This incoherent processor was using a matrix of LEDs as input plane and a matrix of kinoforms as processing plane. It was used successfully in a real high-energy physics experiment in Brookhaven [124, 125]. The Hough transform is a space-variant operation for detecting the parameters of curves [126]. This transform can take fully advantage of the parallelism of the optical implementation. In 1986, Ambs et al. constructed an optical processor based on a matrix of 256 Ã- 256 optically recorded holograms [18, 50]. This implementation was improved ten years later with the use of a large scale DOE composed of a matrix of 64 by 64 CGHs with 4 phase levels fabricated by lithographic techniques [127]. Advances in Optical Technologies 9 Several other optical implementations of the Hough transform were published. Casasent proposed several different optical implementations for example one using acousto optics cell [128]. A coherent optical implementation of Hough transform has been discussed by Eichmann and Dong [129], where the 2D space-variant transfer function is implemented by successively performing 1D spaceinvariant transforms by rotating the input image around its center point and translating a film plane for recording. Another implementation for coherent or incoherent light was proposed by Steier and Shori [130] where they use a rotating Dove prism to rotate the input image, and thedetection is achieved by a linear detector array. Today the Hough transform is widely used in image processing for detecting parametrical curves, but the implementation is electronic. Yu et al. proposed several optical processors for pattern recognition using different types of input SLMs [131]. For example an adaptive joint transform correlator for autonomous real-time object tracking [132], an optical disk based JTC [133]. Pu et al. constructed a robot that achieved real-time navigation using an optoelectronic processor based on a holographic memory [134]. Thomson-CSF in France, in the frame of a European project, constructed and tested successfully a compact photorefractive correlator for robotic applications. The size of the demonstrator was 600mm Ã- 300 mm, it was composed of a mini-YAG laser, a liquid crystal SLM and an updatable holographic BSO crystal [135]. This correlator was also used for finger print identification [136]. Guibert et al. constructed an onboard optical JTC for real-time road sign recognition that was using a nonlinear optically addressed ferroelectric liquid crystal SLM in the Fourier plane [137]. A miniature Vander Lugt optical correlator has been built around 1990 by OCA (formerly Perkin-Elmer). This correlator was composed of a Hughes liquid crystal valve, a set of cemented Porro prisms and a holographic filter. The purpose of this processor was to demonstrate this technology for autonomous missile guidance and navigation. The system was correlating on aerial imagery and guided the missile to its preselected ground target. The processor was remarkable by its rugged assembly; it was 105mm in diameter, 90mm long, and weighted 2.3kg [138]. In 1995, OCA constructed a prototype of an optical correlator that was fitting in the PCI slot of a personal computer and was able to process up to 65Mbyte of image data per second [139]. This processor was intended to be commercialized. The Darpa in the USA launched in 1992 a project named TOPS (Transitioning of Optical Processing into Systems) associating some universities and about ten important companies potential users and developers of the technology. BNS presented in 2004 an optical correlator using four kilohertz analog spatial light modulators. The processor was limited to 979 frames per second by the detection camera. However, the rest of the correlator was capable of 4,000 frames per second [140]. The Jet Propulsion Laboratory (JPL) developed several optical processors for real time automatic target recognition [141]. The University of Sussex constructed also an alloptical correlator and a hybrid digital-optical correlator [142]. It should also be noted that several optical correlators were available commercially but it is not sure that it was a commercial success since most of them are no longer commercialized. For example, Dong [129], where the 2D space-variant transfer function is implemented by successively performing 1D spaceinvariant transforms by rotating the input image around its center point and translating a film plane for recording. Another implementation for coherent or incoherent light was proposed by Steier and Shori [130] where they uses rotating Dove prism to rotate the input image, and the detection is achieved by a linear detector array. Today the Hough

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